When cathodic protection is used to protect structures positioned in a water environment, a variety of sacrificial and impressed current anode systems can be utilized. Generally, impressed current anode systems are used for applications where higher amperages are required to achieve the desired level of cathodic protection while sacrificial anode systems, usually made from zinc or aluminum, corrode to produce the electron flow that protects the steel structure from corrosion.
Anodes have a higher efficiency when positioned in the water, above the sea floor and sediment rather than buried beneath.
The term “sea floor” is used to describe the solid surface underlying a body of water such as, for example, oceans, seas, harbors, lakes, estuaries, etc.
Many methods have been described in the prior art which utilize one or more anode(s) to protect metallic underwater structures from corrosion. Anodes positioned on the sea floor are typically mounted on structures constructed from concrete, steel and or fiberglass. These structures are oftentimes referred to as “sleds”. Sleds are utilized to ensure that the anode(s) remain above the mudline or soft sediment deposited upon the sea floor. The sled design may also incorporate features including: a structural attachment for the anode and anode power cable, a mounting point for power cable(s) junction box, or ballast (mass/weight) to eliminate physical movement or overturning in high velocity current environments or during storm events, and protection from fishing equipment or objects that have been dropped from the surface.
For impressed current cathodic protection, the prior art anodes are connected to one or more cables, and because of the shape and construction of the anodes, the connection to the cables generally must be done in a factory before the anode is mounted to the sled. Typically, the concrete weight and support material must be cast before the anode assembly is shipped from the factory. The requirement to connect the cable and possibly cast the concrete increases the cost and shipping of the sled.
Similarly, for cathodic protection using sacrificial anodes, the design is based upon the surface area of the anode exposed to the water. Cables are then used to connect the sacrificial anode to the subsea structure.
One example of a marine system using sacrificial anodes for cathodic protection is U.S. Pat. No. 6,461,082. A plurality of sacrificial anodes are positioned on top of an elongated electrode carrier, such as a long pipe which is laid upon the seafloor. Each anode is attached to the pipe and the pipe is thereafter operably connected to the structure. In order to maximize the surface area of the anode exposed to the water, this invention requires an additional supporting structure such as a mud mat on the seafloor so that the pipe will not sink into the mud and cover the pipe and anodes.
Another example of a marine system using sacrificial anodes for cathodic protection is U.S. Pat. No. 7,329,336. A plurality of blocks spaced apart and connected by wire rope is used as a seafloor mat. Each block is filled with concrete and at least one of the blocks has embedded in it a sacrificial anode. The anode is embedded so that only the top surface of the anode is exposed for contact with the water. The blocks allow for the concrete to be poured at surface on-location; thus significantly reducing transportation costs associated with loading, hauling and unloading ballast filled blocks. However, because only the top surface of an anode is available for contact with water, surface area is compromised and it is necessary to include many more blocks having the embedded anode to achieve the same level of protection as an anode having substantially its entire surface area exposed to the water.
One example of a marine system using impressed current for cathodic protection is described in U.S. Patent Application Publication No. US 2012/0305386. This anode assembly includes a spherical anode, a vertical anode support structure and a weighted base which is hollow and made of a fiberglass shell. The lower end of the anode support structure is disposed within the hollow base which is thereafter filled with concrete.
Manufacturing and shipping costs for a state-of-the-art “anode sled” are very high, due to the materials employed and the required size of the sled.
Subsea corrosion protection can incorporate the use of fiberglass sled structures to support anode(s) above the seabed as manufactured by Marine Project Management, Inc., Ojai, Calif. (www.mpmi.com). Another method is to utilize a steel structure to support anodes which is basically a framework which lies on the sea floor as manufactured by Deepwater Corrosion Services, Inc., Houston, Tex. (www.stoprust.com). Both types of structures mentioned are expensive to fabricate and ship, and require a significant lead time to manufacture and deliver.
Grout bags have been used for many years for supporting underwater pipeline spans and are intended to be pumped full of cement grout once in place underwater. A typical grout bag is one provided by SEA STRUCT Pty Ltd, North Fremantle, Australia (www.sea-struct.com.au). These devices are more specifically used for supporting an underwater pipeline in areas where the pipe is suspended above the sea floor for a length which could cause the pipeline to buckle. Grout bags are structural in nature and are placed along a spanning pipeline to reduce the span length and are not related to anodes or cathodic protection.